Method of electrolytically chemically treating metals



United States Patent 3,288 691 METHOD OF ELECTROLYTICALLY CHEMICALLY TREATING METALS Shigeru Yonezaki, Minoru Kamada, Kazukiyo Teruyama, and Katsunori Kanechika, all of Kitakyushm-Japan, assignors to Yawata Iron & Steel Co. Ltd., Tokyo, Japan, a corporation of Japan No Drawing. Filed June 11, 1963, Ser. No. 286,918 Claims priority, application Japan, June 13, 1962, 37/23,910 8 Claims. (Cl. 204-56) This invention relates to a method of electrolytically chemically treating metals, and more particularly to a method of cathodically electrolyzing a metal, especially steel sheet and aluminium sheet, in a chromic acid bath having a halogen oxy acid therein in order to instantly obtain a tough and adhesive film on the metal surface.

An object of the present invention is to provide a method of forming on a metal surface a film which is very high in anticorrosiveness and weatherproofness, before as well as after the painting thereof.

Another object of the present invention is to provide a method of forming on a metal surface a film which is so tough and adhesive that it will not peel off even if severely brushed and from which film no paint will peel off even if the metal is severely worked .after it has been painted.

Another object of the present invention is to provide a method of forming a film on a metal surface within a very short time, such as 0.1 to 1 second.

A further object of the present invention is to provide a method of giving a metal surface one of a plurality of various colors such as light brown, bluish brown, blue, yellowish blue, yellow, red and green.

Many other objects and efiects of the present invention will become clear from the following specification.

The present invention is characterized by cathodically electrolyzing a metal in an aqueous solution adjusted to have a pH less than 1.5 by using a hexavalent chromium compound as a main agent and a halogen oxy acid or its salt as an assistant.

The most important technical matter in the requirements of the method constituting the present invention is that halogen oxy acid ions be present in the solution containing the hexavalent chromium compound so that a tough and adhesive film can be formed within a very short time, such as 0.14 second. As the result of the investigations it has been discovered that, when the cathodic-electrolytic treatment is carried out in an aqueous solution of hexavalent chromium ions having no halogen oxy acid ions therein, a film is produced which will easily peel ofl either during the process of said film being formed or during the working of the metal after painting, whereas a film produced by the cathodic electrolytic treatment in a chromic acid electrolytic bath containing halogen oxy acid ions will not peel off even if brushed severely or worked severely after painting.

Because halogen oxy acid ions are so effective in accelerating the formation of an electrolyzed film, an instant formation of the same is made possible by adding halogen oxy acid ions to an aqueous solution of hexavalent chromium acid ions. For instance, when no halogen oxy acid ions are contained in a solution of hexavalent chromium ions, the electrolysis must be continued for more than 20 second in order to obtain any satisfactory film on the metal surface, whereas the same amount of film can be obtained in 1 second if g./l. of a halogen oxy acid salt are added or only in 0.3 second if 20 g./l. are added.

Some acceleration of the film formation will also be produced by halogen ions and aromatic disulphonic acids. However, when using any of them as an assistant to the uneconomical.

Patented Nov. 29, 1966 chromium acid solution, it takes much more time to form the desired film than when using halogen oxy acid ions, and the film obtained thereby is likely to peel off as it is formed or during the working after paint ing. For instance, it will take 1 to 4 seconds to obtain an electrolyzed film when using halogen ions and 20 to 30 seconds when using aromatic disulphonic acids. It has been, therefore, proved that halogen oxy acid ions are far superior in accelerating the film formation to any other assistant. Thus, the film produced by the method of the present invention using halogen oxy acid ions not only excels those produced by other methods using other assistants, but also it may be produced much more quickly than by any other method.

However, the objects of the present invention can not be attained by the presence of halogen oxy acid ions alone. The further requirement for the present invention is the limitation of the pH value of the electrolysis solution. Even when halogen oxy acid ions are present in the chromium acid solution in a sufficient amount and the treating time is extended, the desired film may not be formed if the pH value of the electrolytic solution exceds 1.5. The limitation of the pH value of the treating solution to 1.5 is the second feature of the present invention.

The electrolytic bath used in the present invention consists of hexavalent chromium compound as the main agent and halogen oxy acid or its salt as the assistant. The former comprises chromic anhydride, dichromates and chromates. However, chromic anhydride is usually used, and chromium ions may be obtained by dissolving said chromic anhydride in water. When using dichromates and chromates, the pH values of their aqueous solutions of them are so high that it will be necessary to adjust the pH values of these solutions by adding an acid such as perchloric acid or nitric acid to them. The amount of the hexavalent chromium compound to be added is 10 to 300 g./l. When it is less than 10 g./ 1., the film produced will lack uniformity and when over 300 g./l., the solution of high concentration will be hard to use in the operation and more over will be Practically, the addition of 20 to g./l. is preferable.

As regards the halogen oxy acid or its salt to be added to the solution as the assistant, an increase in the amount added up to a certain limit may increase the thickness of the film formed by the electrolysis and improve the toughness and adhesiveness thereof. However, if the amount added becomes too much, exceeding the certain limit, the effect of halogen oxy acid ions in accelerating the film formation will not increase and the adjustment of the treating solution will become rather difiicult.

According to the present invention, it has been discovered that an amount of 1 to 200 g./l. is favorable. However, practically 10 to 100 g./l. is preferable.

Halogen oxy acid ions may be provided by adding halogen oxy acid or its salt to an aqueous solution of hexavalent chromium compound, or by making a salt of a halogen oxy acid by the addition of an alkali to an appropriate acid and then adding said salt to an aqueous solution of hexavalent chromium compound. As an example of the former, perchloric acid, potassium chlorate, sodium perchlorate and sodium bromate can be used as halogen oxy acid or its salt, and as an example of the latter, sodium carbonate can be added to perchloric acid and the resulting salt added to the solution of hexavalent chromium compound. The water to be used in preparing the electrolytic solution may be either pure water or tap water. In preparing said solution, the respective ingredients are adjusted so that the pH of the final solution is less than 1.5.

Substantially no close relation is perceived between the temperature of the treating solution and the film to be produced. However, if the temperature of the solution becomes so high that it is near the boiling point of water, the adjustment and control of the treating solution will become difiicult due to the temperature and the concentration of the solution resulting from the evaporation thereof. It is, therefore, desirable to keep the solution temperature at 10 to 90 C.

The anticorrosiveness of the film produced depends upon the amount of deposit making up the film, which is again determined by the amount of halogen oxy acid or its salt to be added, the current density and electrolyzing time. The higher the :current density and the longer the electrolyzing time, the more material will be deposited. Any current density and electrolyzing time may be selected. However, as the current density and electrolyzing time are correlated with each other to obtain the same amount of deposition, the higher current density may be combined with the shorter electrolyzing' time, and vice versa.

Thus, by adjusting the amount of halogen oxy acid or its salt to be added, the current density and the electrolyzing time, a film of any desired thickness can be obtained. It has been proved by the investigations that an amount of material suflicient to form a film of 2 to 3 mg./dm. affords sutficient anticorrosiveness.

Further, the toughness and adhesiveness of the film produced mainly depends on the amount of halogen oxy acid or its salt which is added. As already mentioned, an amount of halogen oxy acid of 1 to 200 g./l. is favorable. 1 g./l. is the necessary lower limit. If less than 1 g./l. is added, the film produced will lack adhesiveness and workability. With an increase in the amount of halogen oxy acid added, the toughness and adhesiveness of the film will be improved accordingly. Though there is no upper limit in the strict technical meaning of the word, 200 g./l. may be said to the upper limit from the economic standpoint.

The color of the treated product varies depending on the amount of material in the electrodeposited film. With the increase in the amount of material in the electrodeposited film the color of the film varies from light brown (-at less than 1 mg./dm. to bluish brown (at 1-2 mg./dm. to blue to yellowish blue (at 2-3 mg./dm. to yellow (at 3-4 mg./dm. to red (at 4-5 mg./dm. and to green (at 5-6 mg./dm. Therefore, the present method may be utilized simultaneously as a method of coloring metals. However, in practice, by performing the electrolytic treatment so as to be able to produce a blue or yellowish blue color (at 2-3 mg./ dmfi), a film can be obtained which is sufiicient in anticorrosiveness, paint adhesiveness, workability after painting and anticorrosiveness after painting and working.

After the electrolytic treatment, the product must be dried. The product may be squeezed by rolls and then dried to dry up the remaining solution, or may be washed with water to wash away the solution.

As a pretreatment, the degreasing of material is always required. However, if the material is not rusted, pickling thereof is not always needed.

If, after the electrolytic treatment, the product is washed with water and then is after-treated by being dipped in a dilute alkali solution such as, for example, a solution of 1% by weight sodium carbonate, of 1% by weight sodium hydroxide or of 3% by weight ammonia, the anticorrosiveness will be further improved.

The composition or structure of the film obtained by the present invention is not yet fully known. However, the results of the investigations by electron beam diffraction or with an electron microscope have indicated the fact that neither crystalline film or metallic chromium was found to be present. From this fact it is presumed that the film produced by the present invention would be in an amorphous gelatinous state and that the mechanism of the film being formed by the present invention, wherein a metal is cathodically electrolyzed in a solution containing hexavalent chromium ions, would be as follows: i.e., said hexavalent chromium ions would be reduced to trivalent state on the metal surface and at the same time the pH would rise on the surface and thereby said chromium ions would be deposited on the metal surface in a gelatinous state as chromium chromate.

When carrying out the cathodic electrolysis continuously for a long time, some trivalent chromium ions might be produced in the solution. Trivalent chromium ions which are not deposited on the metal surface but remain in the solution have no influence on the electrolytic treatment, but they will rather contribute to facilitate the drying of the film when the remaining solution is dried up after the film produced is squeezed by rolls. In what part of the process of the film formation halogen oxy acid has a catalytic effect and why a tough and adhesive film is thereby formed are not yet clear. However, the fact is that when no halogen oXy acid ions are contained in the solution, the deposition of chromium chromate will be very difficult and the produced film will be brittle, but that by adding halogen oxy acid ions to the solution the film formation will be greatly accelerated and the film produced will become very tough and adhesive.

The thus produced film will not only not peel off even if severely brushed, but also will better adhere .to the metal surface when dried, will become more tough at the same time and will be water-repellent and will grow in anticorrosiveness with the lapse of time.

Examples of the present invention shall be given in the following:

Example 1 An electrogalvanized steel sheet was cathodically electrolyzed under the following conditions:

Chromic anhydride g./l 15 Sodium perchlorate g./l 3

Treating solution temperature C 40 Cathode current density a./dm. 15

Electrolyzing time seconds 1 Example 2 An aluminum sheet was treated with a dilute alkali solution, was washed with water and was then cathodically electrolyzed under the following conditions:

Chromic anhydride g./l 20 Potassium chlorate g./l l0 Treating solution temperature C 30 Cathode current density ..a./dm. 10

Electrolyzing time seconds 1 Example 3 A steel sheet was electrolytically degrea'sed with an aqueous solution of an alkali, was washed with water and was then cathodically electrolyzed under the following conditions (also in Examples 4, 5, 6 and 7):

A steel sheet prepared as in Example 3 was cathodically electrolyzed under the following conditions:

Chromic anhydride 'g./l 20 Potassium chlorate g./l 5 Treating solution temperature C 60 Cathode current density a./dm. 10

Example 4-Continued Electrolyzing time seconds 1 pH 0.7

Example 5 A steel sheet prepared as in Example 3 was cathodically electrolyzed under the following conditions:

Chr-omic anhydri'de g./l 50 Perchloric acid g./l 100 Sodium carbonate g./l 5'0 Treating solution temperature C 50 Cathode current density a./dm. 6 Electrolyzing time seconds 0.3 pH 0.6 Example 6 A steel sheet prepared as in Example 3 was cathodically electrolyzed under the following conditions:

Chromic anhydride .g./l 10 Potassium bromate g./l 8 Treating solution temperature C 60 Cathode current density a./dm. 20 Electrolyzing time seconds 0.4 pH 0.8

The product was after-treated with a solution of 1% sodium carbonate.

Example 7 A steel sheet prepared as in Example 3 was cathodically electrolyzed under the following conditions:

C-hromic anhydride g./l 40 Perc'hloric acid g./ l 20 Sodium chloride 'g./ l 0.05 Treating solution temperature C 60 Cathode current density a./dm. '15 Electrolyzing time seconds 0.4 pH 00.3

A slight amount of halogen ions was present.

The product was after-treated with a solution of 0.3% ammonia.

When the steel sheets treated under the above mentioned conditions according to the present invention were left indoors, they did not rust even in 6 months, even when tested in a humidity cabinet at a humidity of 100% and a liquid temperature of 49 C. for one week and even when tested with a dew point corrosion tester of an outer tank at 49 C. and an inner tank at 39 C. for one week. Further, when they were subjected to a salt water spray test according to JISZ 2371, wherein salt water having a concentration of 5% was sprayed against a wall through a nozzle at a tank temperature of 35 C. and a test piece angle of degrees so that the rising salt water mist was sprinkled on the surface of the test piece, no rusting was seen even after the lapse of five hours.

In the same salt water spray test, a steel sheet treated with a phosphate and a steel sheet not treated at all had a red rust on the whole surface in about 30 minutes and in about 5 minutes, respectively. Further, the steel sheets treated under the conditions mentioned in the above examples had the same paint adhesiveness as did the phosphate treated steel sheet.

For example, with the treated steel sheet, after painting when the painted surface was scratched and was subjected to the salt water spray test, the painted film did not blister even in 10 days. On the other hand, with the untreated steel sheet, all the painted film peeled ofi? in one day in the same test.

As regards the workability after painting and the anticorrosiveness after painting and working, even when the treated steel sheet was painted with a paint of a me lamineurea resin series, was baked and was subjected to an impact test of a Du Pout impact tester wherein the punch diameter was inch, the load was 1 kg. and the falling height was 50 cm., the painted film was never seen to peel oif or crack. When the same test was applied to the steel sheets electrolyzed with the treating solution having no assistant and with the treating solution containing only halogen ions as an assistant, the films peeled oh? the boundary between the film and the surface of the steel sheet at a falling height of 30 cm. Further, when the steel sheet was painted with an internal surface paint of a phenol series for use in canning, was baked and worked to make a lid of 301 mm. in diameter, the painted film did not peel off at all. But, when halogen ion-s and aromatic disulphonic acids were used as assistants, the painted film peeled off sometimes. Further, when a steel sheet electrolyzed in the electrolytic solution according to the present invention, was worked to make the same lid, no rusting was seen even after a lapse of 24 hours in the same salt water spray test. On the contrary, with the untreated steel sheet, the steel sheet electrolyzed with chromic acid when using only the halogen ions as an assistant and the steel sheet electrolyzed with chromic :acid when using the aromatic disulphonic acid as an assistant, a red rust was seen to be produced at rusting area rates of 10, 3 and 3% respectively in the same salt water spray test.

What is claimed is:

1. A method of electrolytically chemically treating metals comprising depositing a film on the metal by cathodically electrolyzing a metal in an aqueous solution consisting essentially of 10 to 300 grams per liter of 'hexavalent chromium compound and 1 to 200 grams per liter of a member selected from a group consisting of halogen oXy-acids and their alkali metal salts said solution being adjusted to have a pH value of less than 1.5.

2. The method as claimed in claim 1, wherein the hexavalent chromium compound is ch-romic anhydride.

3. A method as claimed in claim 1, wherein the pH is adjusted by adding to the solution a member selected from a group consisting of perchloric acid and nitric acid.

4. The method as claimed in claim 1, wherein the electrolysis is carried out to deposit material in the film in an amount of from 1 to 6 milligrams per dm.

5. The method as claimed in claim 1, further comprising adding 0.05 :gram per liter of sodium chloride to the aqueous treating solution.

6. The method as claimed in claim 1, wherein the electrolysis is carried out for a period of from 0.1 to 1 second.

7. The method as claimed in claim 1, wherein the amount of hexavalent chromium compound present in the solution is from 20-100 grams per liter.

8. The method as claimed in claim 1, wherein the amount of the member from said group is present in the solution in an amount of from 10 to grams per lite-r.

References Cited by the Examiner UNITED STATES PATENTS 2,769,774 11/1956 Loveland et al 204-56 2,780,592 2/ 1957 Wick et al. 204-56 2,812,296 11/1957 Neish 20456 X 2,998,361 8/1961 Kiitam-ura 20456 3,032,487 5/1962 Yonezaki et al. 20456 3,081,238 3/1963 Gurry 20456 3,118,824 1/1964 Yonezaki et al. 204-56 3,138,548 6/1964 Ham et al. 20456 X JOHN H. MACK, Primary Examiner.

G. KAPLAN, Assistant Examiner. 

1. A METHOD OF ELECTROLYTICALLY CHEMICALLY TREATING METALS COMPRISING DIPOSITING A FILM ON THE METAL BY CATHODICALLY ELECTROLYZING A METAL IN AN AQUEOUS SOLUTION CONSISTING ESSENTIALLY OF 10 TO 300 GRAMS PER LITER OF HEXAVALENT CHROMIUM COMPOUND AND 1 TO 200 GRAMS PER LITER OF A MEMBER SELECTED FROM A GROUP CONSISTING OF HALOGEN OXY-ACIDS AND THEIR ALKALI METAL SALTS SAID SOLUTION BEING ADJUSTED TO HAVE A PH VALUE OF LESS THAN 1.5. 